It's truthfully extremely hard to balance slice of life and romance in one manga, especially if you have as many chapters as Amano Megumi. But the thing is right, if you want to make it mostly slice of life, you have got to make it entertaining - make it funny, use it to world-build, maybe even try different types of comedy to test the waters - just do something.
Let's look at Komi-san. It's already been 200 chapters, yet Tadano and Shouko have not started dating, or gone through the rough period before dating. It's, quite literally, 90% slice of life, 10% romance. The thing about it, is that the slice of life has a purpose: to build the world and its characters. Komi-san slice of life is usually never about ONLY Tadano or Komi. It's always them and their friends or family, or just their friends. This makes it interesting and worth reading, since the slice of life has a use. Also, Komi's cute. Tadano, although debate-ably a self-insert, is an endearing character. The rest of them are also pretty good. Having a wide range of characters means you have a big bag of jokes, so they don't get stale easily.
In Amano Megumi, it's just Shindo trying to study and Amano flashing either her tits or ass. I'm not going to criticize fanservice, but nobody reads 200 chapters of fanservice to beat their dick. That's what hentai is for. Ecchi is fine, if used correctly, which is not the case here. The best way that I can describe it is: boring. The side characters are boring and one-dimensional, hell even the two main characters are barely two-dimensional.
"But Komi-san also has one-dimensional characters!" In Komi-san's case, it's not a matter of inadequate writing, but a purposeful choice. Tadano Hitohito's name literally means "just a guy", referring to the fact that when put side-by-side with the beautiful schoolgirl Shouko, he seems painfully average in comparison. Osana Najimi's name literally means "childhood friend", which is funny since he is a childhood friend to probably the entire country of Japan. It's funny, and even if they have a single base, it doesn't become a crux - they're still entertaining even in their own archetypes. Tadano's normal, but because of this, he appreciates everyone around him. Najimi has a lot of friends, so he picks up on subtle cues easily, and so on and so forth.
In conclusion, I don't consider Amano Megumi a "bad" manga, since it depends on what you are reading it for. If you just want a one-chapter-a-day slice of life, similar to 60% of all anime in existence, then sure, by all means, there's ecchi too so you're entertained on top and down there. But if you to a 100+ chapter romance looking for a hefty supporting cast, main characters with depth, and interesting character dynamics, this isn't for you.
I don't like slice of life that's boring. "LIfe's boring, deal with it." Why would I read media that provides me the same amount of entertainment as my own life? I come here to read interesting stories and look at good art; if I wanted to see actual life I'll just watch the news or something.
I'd like to compare SoL manga to food. Komi-san is a meal plan where every dish is different - most of the time they aren't anything too special, but once in a while something good comes out, and the variety and surprise jump in quality keeps your palate refreshed. Amano Megumi on the other hand, is like an endless line of just sushi. Sure, it's good the first week or two, but even eating something as delicious as sushi will get boring if consumed day in and day out. I like sushi if you couldn't tell so just substitute it with your favourite food if you don't get it.